The present invention relates to a belt structure of a pneumatic radial tire and more particularly to a heavy duty and medium pneumatic radial tire improved in not only the durability of the crown portion on a rough road through the formation of a split structure in the first belt layer but also the dimensional stability and the durability of the belt layers during travelling on an expressway.
A conventional heavy duty and medium pneumatic radial tire adapted for use in trucks and buses or light trucks comprises, e.g., as shown in FIG. 5, a multiple belt layer 4 disposed between a tread 2 and a carcass layer 3 of a tire 1, said belt layer 4 being composed of a belt reinforcing layer 4.sub.1 (the first belt layer) having a cord angle of 40.degree. to 75.degree. relative to the circumferential direction of the tire disposed adjacent to the carcass layer 3 and, superimposed on the belt reinforcing layer 4.sub.1, at least two belt tension-resistant layers 4.sub.2 and 4.sub.3 (the second and third belt layers) each having a cord angle of 10.degree. to 30.degree. relative to the circumferential direction of the tire and disposed so as for the cords constituting the second belt layer to cross those constituting the third belt layer. In this case, the carcass layer 3 has a monolayer of multi-layer structure. The cords of the carcass layer are each provided at an angle of about 90.degree. relative to the circumferential direction of the tire (i.e., substantially in a radial direction).
In a radial tire shown in FIG. 5, the dimensional stability is ensured by disposing the belt reinforcing layer 4.sub.1, i.e., the first belt layer, over substantially the entire region of the crown portion in order to reinforce the crown portion so as to withstand the inflation pressure of the tire, and an excellent effect of improving uneven wear resistance and driving stability is attained by enhancing the sectional bending rigidity in a radial direction of the tire (i.e., a widthwise direction of the tire) over the entire ground-contacting area of the tread. However, it is difficult for this tire to trace changes in the profile of the road surface having unevennesses, such as stones or protrusions, which brings about problems that the stress concentration attributed to the unevenness of the road causes the central region of the crown to be damaged and, further, the cords of the belt layer within the tire to be broken.
In order to eliminate these drawbacks, a tire having a structure shown in FIG. 6 has been adopted mainly for use on a rough road. In this tire, the belt reinforcing layer 4.sub.1 is removed from the central portion of the crown and divided into two parts to dispose them respectively on both shoulder portions, i.e., to form a split structure, thereby imparting flexibility to the central region of the crown susceptible to stress concentration and relaxing the stress through a lowering in the sectional bending rigidity in the radial direction of the central region of the crown. Although this split structure hardly brings about any problem in the case of a tire for use on a rough road having a short travel life, it makes the shape of the crown portion unstable (particularly brings about an increase in the growth of the outer periphery of the central portion of the crown) due to a lowering in the belt reinforcing function of the central portion of the crown. This brings about another problem that the strain between the tension-resistant layers 4.sub.2 and 4.sub.3, i.e., the second and third belt layers, is gradually increased and finally leads to the occurrence of separation at the end portion of the belt layer.
On the other hand, as described in Japanese patent application Kokai publication No. 61-165514, a proposal has been made on a technique for ensuring the durability of the belt portion under travel conditions on an expressway through the provision of an organic cord layer in a circular form in the circumferential direction of the tire at an angle as low as 0.degree. to 10.degree. relative to the circumferential direction of the tire in the space formed by splitting the first belt layer for the purpose of ensuring the dimensional stability. Although the tire prepared by the above-described technique brings about no problem under permissive load conditions, it brings about a problem under remarkably high load conditions that the driving stability is lower than that of the conventional tire wherein the first belt layer is disposed over substantially the entire region of the crown portion.
In the above-described tire having an organic fiber cord layer disposed on the carcass layer, a lift of the tread portion occurs during vulcanization, so that the splice of the organic fiber cord layer is slipped in the circumferential direction of the tire. The slippage unfavorably brings about the occurrence of cord waves in the carcass layer. Specifically, when an unvulcanized tire is placed in a mold for vulcanization molding, a lift occurs usually with a percentage lift of 2 to 4%. The belt layer copes with the occurrence of the lift through a change in the cord angle and an increase in the cord intervals for elongation in the circumferential direction of the tire. However, it is impossible to take such a measure when the organic fiber cord layer is provided at a cord angle of 0.degree. or so. For this reason, the following measures are taken in the organic fiber cord layer: (1) elongation of the cords themselves through the tension of the cord caused by the lift and (2) slippage, in the circumferential direction of the tire, of each end portion of the splice formed by bonding one end of the organic fiber cord layer to the other end thereof through polymerization during molding of the tire.
However, with respect to the above-described measure (1), since the cord tension applied to the organic fiber cord layer during vulcanization is 2 to 3 kg/cord, an organic fiber exhibiting a high elongation should be used in order to cope with a percentage lift of 2 to 4% through elongation of the cords themselves, which brings about a lowering in the reinforcing effect of the organic fiber cord layer, i.e., makes it impossible to attain the primary object of the use of the organic fiber cord layer. With respect to the above-described measure (2), when the degree of the slippage is large, a shearing force accompanying the shift of the organic fiber cord layer in the circumferential direction of the tire is applied to the carcass layer, so that the cords of the carcass layer disposed in the radial direction brings about a change in the cord angle in a wavy form at the splices because at that time the tire is in an unvulcanized state. These phenomena are shown in FIGS. 7(A), (B), (C), and (D). FIG. 7(A) is a cross-sectional view of a splice of an organic fiber cord layer in an unvulcanized state; FIG. 7(B) is a plain view of the splice shown in FIG. 7(A); FIG. 7(C) is a cross-sectional view of the state of the slippage of a splice of an organic fiber cord layer after vulcanization; and FIG. 7(D) is a plain view of the splice shown in FIG. 7(C). In FIGS. 7(A) and (B), an organic fiber cord layer 5 is disposed on a carcass layer comprising carcass cords 3a. Letter a designates a splice. After vulcanization, as indicated by a dotted line in FIG. 7(C), the splice of the organic fiber cord layer 5 on the carcass layer side is slipped in a direction shown by an arrow due to the lift during vulcanization, which brings about relative shift of the laps of the splices. As shown in FIG. 7(D), this in turn causes the carcass cords 3a to undergo a change in the cord angle in a wavy form at the splice thereof.
When the vulcanization is conducted in such a state that the cord angle is changed in a wavy form, a carcass cord wave is formed, so that when the tire is filled with air, there occurs a force causing the carcass cords to be radially orientated due to the tension. This unfavorably brings about an increase in the ply steer force and a lowering in the durability of the belt layer.
In particular, a large degree of slippage in the circumferential direction of the tire at the splice occurs when the organic fiber cord layer has a single layer structure. When the organic fiber cord layer is formed to have a double layer structure, the degree of slippage can be reduced to such an extent that the above-described adverse effect becomes negligible. However, in this case, since the degree of slippage is remarkably small, the elongation of the organic fiber cord layer should be increased in order to cope with the lifting. This brings about not only an increase in the growth of the outer periphery of the tire after vulcanization but also a lowering in the durability of the belt portion.